1. Field of the Invention
The present invention relates generally to optical disc media, and more particularly to improving the bonding ability of plastic layers of a multi-layered optical disc by exposing the plastic layers to a plurality of ions.
2. Description of Related Art
It is well known to use optical discs for data storage purposes. Recently, the need for greater storage capabilities has spurred interest in ways to store even greater amounts of data on optical discs. As a result, multi-layered optical discs have been developed. Examples of such multi-layered optical discs include, but are not limited to, Digital Versatile Discs.
Digital Versatile Discs are comprised of two plastic layers that are formed from a polycarbonate material and that are bonded to other portions of the optical disc, typically by use of an ultra-violet curable resin. Because polycarbonate is a hydrophilic plastic, the plastic layers will expand or shrink depending on the amount of moisture in the atmosphere. If one plastic layer shrinks or expands by a different amount than the other plastic layer, it may cause warping of the disc, thereby making the disc difficult, and in certain cases, impossible, to read by optical disc readers. To alleviate this problem, many multi-layered optical discs are manufactured so that the optical discs are symmetrical from a moisture absorption point of view. The two plastic layers essentially shrink or expand the same amount so that the amount of shrinkage or expansion by one of the plastic layers is equal to the amount of shrinkage or expansion of the other plastic layer. This shrinkage or expansion in each direction effectively cancels out, and therefore prevents warping of the disc.
Next generation multi-layered optical discs are being developed to hold even greater amounts of data, but the format of some of these next generation discs is not symmetrical (with respect to moisture absorption). Thus, polycarbonate material cannot be used in these next generation discs because of the disc warping problem described above. Moreover, other next generation optical discs are being developed that are designed to be read by light having lower wavelengths, including lights having wavelengths that approach 400 nm. Polycarbonate material has a low transparency, which means that polycarbonate material absorbs more light at lower wavelengths than other materials that have a higher transparency, such as cyclic-olefin material. As a result, optical discs having layers comprised of polycarbonate material cannot be read in players that use lower wavelength read lights, such as lights having a wavelength of 400 nm, as well as optical discs having layers comprised of other materials having a higher transparency. As a result, other materials having a higher transparency, such as cyclic-olefins, are often used in these other next generation discs, instead of polycarbonate.
Next generation optical discs often utilize substrates comprised of cyclic-olefin materials, such as Zeonex™ and Zeonor™ from Zeon Chemicals, and Topas® from Ticona, which is a division of Celanese AG. Cyclic-olefins are being used in multi-layered optical discs because they do not have the moisture absorption characteristics of polycarbonate, and because they have a higher transparency than polycarbonate. A drawback of using cyclic-olefins, however, is that they do not bond with adhesive materials well. As a result, the layers of multi-layered optical discs that are manufactured from cyclic-olefin separate easily, and the discs therefore have an increased susceptibility to delamination, thus rendering such discs essentially worthless.
Prior methods to improve the bonding ability of plastic layers of multi-layered discs to adhesives included exposing the plastic layer to an oxygen plasma by the use of a point source of the plasma. Although this method did improve the adhesive qualities of the optical disc, it presented several other problems and inefficiencies. For example, because a reactive plasma was used, the plasma overly degraded the surface of the substrate, causing undesired optical degradation of the read light when the disc was played in optical disc readers. Such optical degradation drastically diminishes, and in certain cases, completely destroys, the utility of the optical disc. Moreover, by using a point source to apply the ion plasma, the point source must be moved back and forth in a radial direction relative to the optical disc to treat areas of the disc at different radial distances from the center of the disc. The optical disc must also be rotated so that the point source can treat other portions of the disc. This process is difficult to control, because of the reactive plasma, and it is expensive and time consuming because of the inefficient point source used to expose the plastic layer to the plurality of ions.
Thus, there remains a need for a method of increasing the bonding ability of plastic layer(s) of multi-layered optical discs efficiently, and without overly degrading the surface of the plastic layer of the optical discs.